Fault Current Limiter

ABSTRACT

A fault current limiter (FCL) includes at least one magnetisable core member and at least one AC magnetomotive force source configured to generate a varying magnetic flux in at least a portion of the at least one magnetisable core member. At least one static magnetomotive force source is positioned to provide a magnetic circuit within at least part of the at least one magnetisable core member and the AC magnetomotive force source and the static magnetomotive force source are relatively positioned to be orthogonal to each other. Typically the static magnetomotive force source may be a permanent magnet and the AC magnetomotive force source configured to generate a varying magnetic flux in both of first and second spaced core members.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from PCT Application No.PCT/GB2012/052120, filed Aug. 30, 2012 and from GB 1115005.9 filed onAug. 31, 2011, both of which are hereby incorporated by reference hereinin their entirities.

1. FIELD OF THE INVENTION

Field of present invention relates to a fault current limiter and inparticular a fault current limiter having a re-settable staticmagnetomotive force source.

2. STATE OF THE ART

A Fault Current Limiter (FCL) is a device used to limit, or in its mostbasic form interrupt, a fault current in a branch of a circuit onoccurrence of a fault condition so as to prevent any components in thecircuit from being overloaded.

Fuses are an example of a device which interrupts high currents in faultconditions, however these devices must be replaced after a faultcondition has occurred and cannot be used in high power systems. It isusually more preferable to employ a re-settable FCL which limits thefault current, rather than interrupts it.

An example of a re-settable FCL which is suitable for low poweroperations is the Magnetic Current Limiter (MCL) which comprises apermanent magnet sandwiched between a saturable core with an AC wirewound around the core (see FIG. 1). The permanent magnet 2 causes thecore to saturate in the normal operating state. For the device tooperate for each half of the AC cycle, two cores are required such thatin the first core the magnetic field produced by the AC current flowsthrough the coil since the magnetic field provided by the permanentmagnet are additive and in the second core they are subtractive. In thenormal operating condition the AC current flowing through the coil islow and both cores are saturated causing the effective impedance in theAC coil 3 to be low. During a fault condition a large AC current value(the fault current) forces each of the cores of the device to come outof saturation in alternative half-cycles. The mostly unsaturated firstcore in combination with the mostly saturated second core (and viceversa) restricts the flow of the fault current since the inductance ofthe coil is caused to increase. In this arrangement multiple distinctcore elements are used and useful regions of the core, where interactionbetween the magnetic field associated with the magnet and the AC coiltakes place, are limited. Further, the MCL does not perform well in highpower alternating systems.

In higher power alternating power systems, series reactors have beenimplemented so as to protect against excessively large currents undershort-circuit, however they have a major disadvantage in that theyproduce significant I²R losses.

Another system suitable for use in a high power alternating system isthe saturated Iron Core FCL which comprises a copper coil with an ironcore. The Iron core is maintained in magnetic saturation in normaloperation by applying the magnetic field of an additionalsuperconducting wire. The impedance of the device is low in normaloperation; however when a fault condition occurs the increased ACcurrent through the normal conducting coil causes the core to departfrom saturation so as to cause the impedance of the device to increase.In this arrangement the superconducting wire is exposed only to DCcurrent and therefore always remains in the superconducting state andeliminates the need for a recovery time. The main disadvantages withthis system includes the large mass and volume of the device, the highmagnetic fields at the superconducting coil and the high cooling costsof the superconducting coil.

A recently developed system for use in high power applications is theSuperconducting FCL which relies on a rapid change of resistance withtemperature so as to limit the fault current. The superconducting FCL isdirectly connected in series with the current path to be protected. Whena specified current density is reached, which corresponds to aparticular temperature, the resistance increases rapidly so as tosubstantially limit the flow of the fault current. Such arrangementshave an array of disadvantages including: a) expensive coolingmechanisms since the superconductor must be cooled to 77K, b) thedevelopment of thermal instabilities and c) AC current cooling losses.Further, in order to prevent the excessive heating of thesuperconductor, so as to avoid long cool down phases, the reduced faultcurrent is only be carried for a few cycles.

WO200702924 discloses a fault current limiter device in which anelectromagnet DC source is utilised.

Due to the costs associated with the Saturated Iron Core FCL and theSuperconducting FCL such systems are not usually desirable for smallerpower operations e.g. circuits implementing power electronics devicessuch as transistors, diodes etc.

SUMMARY OF THE INVENTION

It is therefore desirable to provide an improved Fault Current Limiterwhich addresses at least some of the above described problems and/orwhich offers improvements generally.

In a first aspect of the present invention there is provided a faultcurrent limiter (FCL) comprising:

at least one magnetisable core member;

at least one AC magnetomotive force source configured to generate avarying magnetic flux in at least a portion of the at least onemagnetisable core member; and;

at least one static magnetomotive force source being positioned toprovide a magnetic circuit within at least part of the at least onemagnetisable core member,

characterised in that the AC magnetomotive force source and the staticmagnetomotive force source are relatively positioned to be orthogonal toeach other.

In one realisation of the invention, a first core member is provided,and a second core member is provided, spaced from the first core member,the AC magnetomotive force source configured to generate a varyingmagnetic flux in both of the first and second core members.

It is preferred that in such a realisation, the first core membercomprises a first closed magnetic circuit and the second core membercomprises a second closed magnetic circuit distinct from the firstclosed magnetic circuit.

This has an effect that during a fault current event, flux linkage iscompleted predominantly through the AC coils, through the cores, andmagnetomotive force source (e.g permanent magnet(s)), not through theair outside the AC solenoid which is the case in some prior art systems.Effectively, this provides a closed magnetic structure for the ACcircuit. The AC component of the field is encouraged to flow around theclosed circuit.

The first and/or second magnetic circuit may be comprised of coremembers (for example segmented core members) spaced by air gaps. Thishas the benefit that the magnetic flux is inhibited from shortcircuiting in the soft magnetic cores. An air gap may also be insertedin the circumferential path of the soft magnetic cores so as to increasethe reluctance to the AC flux of the soft magnetic core with minimaleffect on the DC path reluctance. This extends the upper range of theFCL before very large AC fields from driving the core back tosaturation.

It is preferred that the static magnetomotive force source is positionedto provide magnetic saturation in both of the magnetic circuits.

It is preferred that the static magnetomotive force source is positionedto bridge the gap between the first and second core members.

Beneficially, the static magnetomotive force source is positioned toprovide a bifurcated magnetic field in the core adjacent to the staticmagnetomotive force source in which the field direction in the same coremember branches in opposed directions at the junction with the staticmagnetomotive force source. This bifurcation of the static magneticfield at the junction with the core members, particularly wherepermanent magnets are used to bridge the gap between core members,provides protection against de-magnetisation of the static magnetomotiveforce source, during a fault current event. The arrangement providescommon mode rejection of the AC field across the poles of the staticmagnetomotive force source.

Preferably the AC magnetomotive force source is an AC coil having alongitudinal axis and to which is applied an AC current so as to producean AC magnetic field and the static magnetomotive force source is amagnet having a magnetic dipole moment associated with it.

Preferably the AC coil and the at least one magnet are relativelypositioned such that the longitudinal axis of the coil is orthogonal tothe magnetic dipole moment of the magnet. This beneficially protects thepermanent magnets and aids saturation of the core material since theDC/static magnetomotive force can be more easily distributed withoutincreasing the AC reluctance of the core material (since the relativepermeability of the permanent magnetic material is low and is seen bythe AC magnetomotive force as a high reluctance element in the magneticcircuit).

Preferably the arrangement of the at least one magnet causes at leastpart of the at least one core member to become saturated in normaloperation wherein in a fault condition the magnitude of the AC magneticfield increases from a normal state value to overcome saturation in atleast part of the at least one core member so that the AC coil obtains ahigher inductance when a fault condition occurs so as to prevent thepassage of the fault current through the coil.

Beneficially the at least one magnet is formed from a permanent magneticmaterial so as to provide a static magnetic (DC) field. The use ofpermanent magnets can be extremely beneficial in that there is norequirement to use superconducting systems or electromagnets thatrequire significant power supply. The concern with the use of permanentmagnets is their potential de-magnetisation during a fault current eventand the result that the FCL would thereafter not re-set to a usablestate. This concern is ameliorated by the way in which the permanentmagnets are incorporated into the magnetic circuit of the device.

During a fault event the flux due to the mmf of the AC coil adds to andsubtracts from the alternate sections or segments of the core members.Flux linkage is largely completed through the permanent magnets in thedirection of pre-magnetised moment of the magnets. This flux linkagethrough the permanent magnet provides protection againstdemagnetisation. The permanent magnets being in closed magnetic circuitprovides protection against demagnetisation. The limit of the faultcurrent limiter will be when the flux due to the mmf of the AC coildrives all segments of the cores beyond saturation.

It is preferred that the static magnetomotive force source is positionedto bridge the gap between the first and second core members.

A benefit of the having magnets bridging the gap is that DC mmf can bedistributed around the magnetic circuit without introducing a break thecircuit of the cores. Further, since the magnetomotive force from thepermanent magnet is more limited than for example a superconductingsource, the magnetomotive force source may be augmented by distributinga number of pairs of magnets, again with no break in the magneticcircuit.

It is preferred that at least two static magnetomotive force sources areprovided, preferably comprising permanent magnets and positioned tobridge the gap between first and second core members. It is preferredthat one or more pairs of magnetomotive force sources are provided,preferably comprising permanent magnets and positioned to bridge the gapbetween first and second core members.

The benefit of such an arrangement is that a low reluctance DC flux pathis provided to aid saturation and protection of the magnets, whilst atthe same time allowing high levels of flux linkage through the magneticcircuit to give good current limiting inductance during a fault. Theability to include magnets distributed in this way provides distributedDC mmf around the magnetic circuit, aiding core saturation.

Beneficially the at least one magnetisable core member responds equallyto the positive and negative halves of the AC current cycle received bythe AC coil such that separate and distinct core devices for dealingwith each half of the AC cycle (as in FIG. 1) are not required.

Preferably the permanent magnetic material of the at least one permanentmagnet recovers its original magnetised state on cessation of a faultcurrent event so as to automatically reset in preparation for thedetection of the next fault current event.

Preferably the magnetisable core contains at least one air gap so as tospace apart the permanent magnets so as to prevent the magnetic fluxfrom short circuiting between the magnets which has the effect ofreducing the flux in the soft magnetic cores. An air gap may also beinserted in the circumferential path of the soft magnetic cores so as toincrease the reluctance to the AC flux of the soft magnetic core withminimal effect on the DC path reluctance. This avoids very large ACfields from driving the core back to saturation. In this arrangement itis preferable for the at least one air gap in the magnetisable core tobe positioned above and below the surface of the pole faces of the atleast one permanent magnet at the region of bifurcation so as to enhancethe performance of the device by extending the working fault currentlimiting range. In an alternative embodiment the at least one magnet isintersected by an air gap so as not to increase the reluctance of the DCflux path, thereby aiding saturation of the soft magnetic cores in theregions between the magnets and also permitting the cores to beconstructed in sections so as to aid the manufacturing process.

Beneficially the AC coil is wound around the region of the magnetisablecore where the overcoming of saturation is to be achieved which reducesthe flux leakage from the soft cores. The length of coil is alsominimised in this arrangement.

Beneficially the AC coil is wound around the region of the magnetisablecore where the overcoming of saturation is to be achieved which reducesthe effect of the AC flux on the cores at normal low currents. Thelength of coil is also minimised in this arrangement. Either position orlength of AC coils may be adopted in the case of optimising the low andhigh current ranges performance of the device.

Preferably the core member is formed from at least two core segmentswhich are arranged to form the core as a whole.

Preferably the FCL can be used for single phase applications or forthree-phase applications.

Beneficially there can be included shielding to minimise the effects ofeddy currents or demagnetisation of the permanent magnet. This,therefore, has the beneficial effect of minimising eddy current heatingand improves the performance of the FCL at higher temperatures, whilstensuring the FCL is reset so as to permit detection of a further faultcurrent event (and prevention of the fault current passing the coil).

Preferably the at least one magnetisable core member is formed of stripsteel or other ferromagnetic metal alloy, a soft ferrite material or anamorphous or nanocrystalline soft magnetic alloy. These materialsprovide the desired saturation capability and can be driven fromsaturation in the event of a fault condition.

Beneficially a circuit or an alternating power system can include thefault current limiter of the invention.

In a second aspect of the present invention there is provided a faultcurrent limiter comprising:

-   -   at least one magnetisable core member having a void defined        therein;    -   at least one conductive element wound around at least a portion        of the at least one magnetisable core member and configured to        receive an AC current to generate an alternating magnetic field        in at least a portion of the at least one magnetisable core        member; and    -   at least two magnets being arranged such that the magnetic        dipole moments of the at least two magnets are opposing, the at        least two magnets being positioned in contact with the at least        one magnetisable core member to provide a magnetic circuit        within the at least one magnetisable core member,    -   characterised in that the conductive element and the at least        two magnets are relatively positioned such that the direction of        the AC magnetic field is substantially orthogonal to the        magnetic dipole moment of the at least two magnets.

Preferably at least two magnets are formed from a permanent magneticmaterial further wherein the at least two magnets are separated by anair gap so as to prevent the magnetic flux from short circuiting betweenthe magnets which has the effect of reducing the flux in the softmagnetic cores. An air gap may also be inserted in the circumferentialpath of the soft magnetic cores so as to increase the reluctance to theAC flux of the soft magnetic core with minimal effect on the DC pathreluctance. This avoids very large AC fields from driving the core backto saturation. In this arrangement it is preferable for the at least oneair gap in the magnetisable core to be positioned above and below thesurface of the pole faces of the at least one permanent magnet at theregion of bifurcation so as to enhance the performance of the device byextending the working fault current limiting range. In an alternativeembodiment the at least one magnet is intersected by an air gap so asnot to increase the reluctance of the DC flux path, thereby aidingsaturation of the soft magnetic cores in the regions between the magnetsand also permitting the cores to be constructed in sections so as to aidthe manufacturing process.

Beneficially the at least one magnetisable core member is formed ofstrip steel or other ferromagnetic metal alloy, a soft ferrite materialor an amorphous or nanocrystalline soft magnetic alloy.

Preferably the arrangement of the at least two magnets causes at leastpart of the at least one core member to become saturated in normaloperation, wherein in a fault condition the magnitude of the AC magneticfield increases from a normal state value to overcome saturation in atleast part of the at least one core member such that the AC conductorobtains a higher inductance when a fault condition occurs.

Beneficially the at least one magnetisable core member responds equallyto the positive and negative halves of the AC current cycle received bythe AC conducting element.

Preferably the at least two permanent magnets recover their originalmagnetised state on cessation of a fault current event.

The second aspect of the invention can be used for single phase or formulti-phase applications.

Beneficially there is included shielding placed around the magnet tominimise the effects of eddy currents or demagnetisation of the staticmagnetomotive force source.

The FCL of the second aspect of the invention may be implemented in acircuit or an alternating power system.

In a third aspect of the invention there is provided a fault currentlimiter comprising:

-   -   at least one magnetisable core member having a void defined        therein;    -   at least one AC magnetomotive force source configured to        generate a varying magnetic flux in at least a portion of the at        least one magnetisable core member; and    -   at least one static magnetomotive force source being positioned        to provide a magnetic circuit within at least part of the at        least one magnetisable core member characterised in that the at        least one static magnetomotive force source is arranged to        extend across the void.

Preferably at least two air gaps are included to section the core memberinto at least a first and second half.

Beneficially the arrangement of the at least one static magnetomotiveforce source causes at least part of the first and second halves of theat least one core member to become saturated in normal operation suchthat saturation in one half of the magnetisable core would be overcomeby the positive half of the AC cycle and saturation in the second halfof the magnetisable core would be overcome by the negative half of theAC cycle.

Preferably the AC magnetomotive force source is an AC coil wound aroundeach half of the magnetisable core which defines the void.

In a fourth aspect of the invention there is a provided a method ofpreventing a fault condition in an AC magnetomotive force sourcepositioned relative to at least one magnetisable core member having atleast one static magnetomotive force source arranged such that the ACmagnetomotive force source and the static magnetomotive force source arerelatively positioned to be orthogonal to each other, the methodcomprising:

-   -   using the static magnetomotive force source to create a magnetic        circuit within at least part of the at least one magnetisable        core member so as to saturate at least part of the magnetisable        core member, thereby providing a low inductance for the AC        magnetomotive force source enabling current to flow along the AC        magnetomotive force source in normal operation;    -   producing an AC magnetic field which overcomes the saturation in        at least part of the at least one magnetisable core member in        the event of a fault condition; and    -   increasing the inductance of the AC magnetomotive force source        in the event of a fault condition so as to limit the passage of        fault current flowing therethrough.

Preferably the fault current limitation responds equally to either halfof the AC cycle.

In a fifth aspect of the present invention there is provided a method ofpreventing a fault condition in an AC conductive element wound around atleast one magnetisable core member having at least two magnets withopposing dipole moment arranged within the at least one magnetisablecore member, the method comprising:

-   -   creating a magnetic circuit within at least one magnetisable        core member;    -   saturating at least part of the at least one magnetisable core        member so as to provide a low inductance for the conductive        element enabling current to flow along the conductive element in        normal operation;    -   producing an AC magnetic field which overcomes the saturation in        at least part of the at least one magnetisable core member in        the event of a fault condition; and    -   increasing the inductance of the conductive element so as to        limit the passage of fault current flowing therethrough,    -   characterised in that the magnetic field generated by the        conductive element is orthogonal to the magnetic dipole moment        of the at least two magnets.

Preferably the fault current limitation responds equally to either halfof the AC cycle.

In a sixth aspect of the present invention there is provided a method ofpreventing a fault condition in an AC magnetomotive force sourcepositioned relative to at least one magnetisable core member having avoid defined therein and having at least one static magnetomotive forcesource arranged to extend across the void, the method comprising:

-   -   using the static magnetomotive force source to create a magnetic        circuit within at least part of the at least one magnetisable        core member, so as to saturate at least part of the magnetisable        core member, thereby providing a low inductance for the AC        magnetomotive force source and enabling current to flow along        the AC magnetomotive force source in normal operation;    -   producing an AC magnetic field via the AC magnetomotive force        source which overcomes the saturation in at least part of the at        least one magnetisable core member in the event of a fault        condition; and    -   increasing the inductance of the AC magnetomotive force source        in the event of a fault condition so as to limit the passage of        fault current flowing therethrough.

Preferably there is included at least two air gaps to segment themagnetisable core into at least a first and second half whereinsaturation in one half of the magnetisable core would be overcome by thepositive half of the AC cycle and saturation in the second half of themagnetisable core would be overcome by the negative half of the ACcycle.

Preferred features relating to the FCL apparatus aspects of theinvention may also comprise preferred features in respect of the methodaspects of the invention.

According to the most broad aspect, there is provided a fault currentlimiter (FCL) comprising:

at least one magnetisable core member;at least one AC magnetomotive force source configured to generate avarying magnetic flux in at least a portion of the at least onemagnetisable core member; and;at least one static magnetomotive force source being positioned toprovide a magnetic circuit within at least part of the at least onemagnetisable core member.

The broadest aspect of the invention can be characterised by one or morepreferred features of the aspects described above or by technicalfeatures described in relation to the specific embodiments which follow.

The present invention will now be described by way of example only withreference to the following illustrative figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Prior Art FIG. 1 shows a side view of a Magnetic Current Limiter inaccordance with the prior art.

FIG. 2A shows a perspective view of a FCL in accordance with a firstembodiment of the present invention and FIG. 2 b shows a detail of FIG.1 explaining bifurcation of the field.

FIG. 3 shows a perspective view of a FCL in accordance with a secondembodiment of the present invention.

FIG. 4 shows a perspective view of a FCL in accordance with a thirdembodiment of the present invention including air gaps arranged betweenthe magnet pair.

FIG. 5 a shows the electric circuit used in the simulations and 5 bshows the results of the simulation of the FCL arrangement of FIG. 4.

FIG. 6 shows a side view of a FCL in accordance with a fourth embodimentof the present invention.

FIG. 7 shows a perspective view of a FCL in accordance with a fifthembodiment of the present invention for use in a single phasearrangement.

FIG. 8 shows a perspective view of a FCL in accordance with a sixthembodiment of the present invention for use in a three phase powersystem.

FIG. 9 shows a perspective view of a section of the core and magnets inaccordance with a seventh embodiment of the invention.

FIG. 10 shows a perspective view of a section of the core and magnets inaccordance with an eighth embodiment of the invention.

FIG. 11 shows a perspective view of the core and magnets arrangedconcentrically

FIG. 12 shows a section of the core and magnets having two outer coreelements and an internal core element.

FIG. 13 shows a first arrangement of conductive plates.

FIG. 14 shows a second arrangement of the conductive plates.

FIGS. 15 a, 15 b and 15 c show a variety of shapes for the magnets inthe FCL.

FIG. 16 shows a perspective view of the FCL in accordance with a ninthembodiment.

FIG. 17 shows a perspective view of the FCL in accordance with an tenthembodiment of the invention.

FIG. 18 shows a perspective view of the FCL in accordance with aneleventh embodiment of the invention.

FIG. 19 shows a wound construction of the core.

FIG. 20 shows the concentric construction of the core.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a magnetisable core 1 formed of two core sections 1 a, 1 b.The first core section 1 a has a picture frame shape and the second coresection 1 b is a mirror image of the first core section. The first andsecond sections 1 a, 1 b are arranged in a face-to-face parallelarrangement, the first core section comprises a first closed magneticcircuit and the second core section comprises a second closed magneticcircuit distinct from the first closed magnetic circuit. A first DCmagnetomotive force source 2 a, for example a magnet, such as a magnetmade from permanently magnetic material (hereafter referred to as apermanent magnet) is arranged to bridge the gap between between the twoopposing faces of the first and second core sections 1 a, 1 b, so as tolink a first arm 1 c of the first core section with a first arm 1 d ofthe second core section. The permanent magnet 2 a is also known as astatic magnetomotive force source and has a magnetic dipole momentassociated with it. It is an advantage of the arrangement of theinvention that for the FCL, re-coil of the permanent magnet is not aprimary concern since the flux linkage through the magnet during faulthelps to maintain the moments of magnets. It is of concern in the caseof the prior art shown in FIG. 1 where the moment of the magnets facefull force of the AC mmf head on. This is required so that the FaultCurrent Limiter can automatically reset following a fault current eventthereby ensuring that the permanent magnet is not permanentlydemagnetised. In this embodiment the permanent magnet is one whichpossesses good re-coil capability such as a rare earth metallic alloy ora hard ferrite. A second permanent magnet 2 b possessing a good re-coilcapability and having a magnetic dipole moment opposing the direction ofthe magnetic dipole moment of the first magnet 2 a is arranged betweenthe two opposing faces so as to link a second arm of the first coresection 1 e and a second arm of the second core section 1 f.

The first and second permanent magnets 2 a, 2 b are in a parallelarrangement and the first and second arms 1 c, 1 d, 1 e, 1 f areparallel arms in the frame arrangement. The flux set up by the first andsecond permanent magnets 2 a, 2 b forms a complete magnetic circuitthrough the soft magnetic material of the core 1 i.e. a magnetic fieldflows from the north pole of the first magnet 2 a through the first coresection 1 a to the south pole of the second magnet 2 b and the northpole of the second magnet 2 b flows through the second core section 1 bto the south pole of the first magnet 2 a. Therefore the magnetic fieldflows in opposite directions in the first and second core sections 1 a,1 b. The relative geometries of the core sections 1 a, 1 b and thepermanent magnets 2 a, 2 b are so as to maximise the ratio of magneticflux interacting to non-interacting volume of the core. Therefore,ideally under normal operating conditions the entire volume of the softmagnetic material of the core 1 remains in the magnetic saturated state.

An AC magnetomotive force source 3, or AC conductive element is woundaround a perpendicular arm of the first and second core 1 a, 1 b in aparallel arrangement so as to provide an AC coil. The AC magnetomotiveforce source 3 and the static magnetomotive force source 2 a,2 b arerelatively positioned to be orthogonal to each other. Therefore thelongitudinal axis of the AC coil is orthogonal (or perpendicular) to thedipole moment of the magnet i.e. the north to south direction of themagnet. Alternatively this can be thought of as the coil being arrangedto provide an orthogonal AC field to the DC field generated in theregion of the core close to the pole face of the DC magnetomotive forcesource (e.g. permanent magnet). The important interaction between the ACand DC fields is where there is a parallel interaction between the ACand DC fields within the soft magnetic cores.

Under normal operating conditions the AC current which passes along theAC coil experiences minimal impedance. As current passes along the ACcoil a magnetic field in a direction perpendicular to the magneticmoment of the first and second permanent magnet is produced. Asmentioned above this may also be defined with respect to the pole facewhereby the AC field in the region near to the pole face of thepermanent magnet is perpendicular to the DC magnetic field generated bythe magnet at the pole face and in the region close to the pole face.

The inductance L of the coil 3 can be approximated with the followingequation:

L=μ ₀·μ_(r) ·N ² ·A/l,

where μ₀—permeability of free space (constant), μ_(r)—relativepermeability of the magnetic core 1, N—number of turns of the coil 3,A—cross section area of the coil 3, l—magnetic path length of the coil3. The N, A and l are linked with the physical design of the coil 3(inductor) thereby making it relatively difficult to change themgradually over a wide range. The permeability of free space μ₀ is aconstant. The relative permeability μ_(r) is a measure of how easy thematerial of the core 1 can be magnetised and it is usually measured forclosed magnetic circuits. The μ_(r) varies with many factors, the mostimportant being the level of magnetisation and for a ferromagnetmaterial this can vary from tens of thousands (at the peak) to one (atextremely high magnetisation).

Therefore, in normal operation the inductance of the AC coil 3 is lowsince the core (comprising of the first and second core sections 1 a, 1b) is saturated by the presence of the magnetic circuit within the corecaused by the magnet arrangement. When a fault condition occurs a highcurrent (a fault current) flows through the AC coil 3 and the magneticfield generated by the AC coil 3 increases in magnitude and becomesstrong enough to overcome the magnetic saturation in regions of the corewhere the AC field and permanent magnetic field interact in oppositedirections, i.e. where the fields are subtractive.

For example firstly considering the effect in the first section of thecore 1 a. When considering the positive half cycle of the AC signal, themagnetic field produced by the AC coil 3 is subtracted from the magneticfield produced by the permanent magnet 2 a in the regions where the twofields oppose causing at least part of the region in the first coresection to become unsaturated. However, when considering the region ofthe first core section 1 a where the fields are in the same directionthe core in this region is driven deeper into saturation, there is nochange in the relative permeability μ_(r) in this region. Therefore thecombined net effect provides a net increase in relative permeabilityμ_(r) which, in accordance with equation 1, provides an increase in theAC coil inductance value. This increase in inductance limits the passageof the fault current through the coil 3.

The second core section 1 b functions in the same way, however thesaturated and unsaturated portions are reversed compared to that of thefirst core section 1 a. This results from the permanent magnetic fieldsin the second core section 1 b being in opposing directions to those inthe first core section 1 a.

When considering the effect of the negative half of the AC cycle on thefirst core section 1 a, the first core section now behaves in the sameway as the second core section 1 b does for the positive half cycle i.e.the regions where the AC magnetic field and the permanent magnetic fieldcoincided previously are now experiencing opposing fields, therefore thetwo fields subtract to give an unsaturated (or less saturated) region,and the regions which were opposing now coincide (adding to give a moresaturated region). The inversion is also applied to the second coreelement 1 b. It is noted that the cyclic nature of the AC current passedthrough the coil 3 causes the direction of the AC magnetic field to vary(or alternate) whereas the magnetic fields caused by the permanentmagnets are fixed in direction, therefore they are said to be DC fieldsor static fields.

Therefore for both halves of the AC cycle (i.e. the positive andnegative parts), the overall effect of the magnetic fault currentlimiter is an increase in inductance of the AC coil 3 due to an increasein μ_(r) of the core (in accordance with equation 1) as a fault currentpasses through the AC coil 3. Therefore the passage of the fault currentcan be limited for each half of the AC current cycle.

The use of permanent magnets for the magnetomotive source elements 2 a 2b can be extremely beneficial in that there is no requirement to usesuperconducting systems or electromagnets that require significant powersupply. The concern with the use of permanent magnets is their potentialde-magnetisation during a fault current event and the result that theFCL would thereafter not re-set to a usable state. This concern isameliorated by the way in which the permanent magnets are incorporatedinto the magnetic circuit of the device. Although the permanent magnets2 a, 2 b may experience brief periods of demagnetisation, they are noteasily permanently demagnetised and ‘spring back’ or ‘re-coil’ into theoriginal (or default) magnetised state following a fault current event.

The static magnetomotive force sources (permanent magnets 2 a, 2 b) arepositioned to provide a bifurcated magnetic field in the core 1 a, 1 bin which the field direction in the same core member branches in opposeddirections at the junction with the static magnetomotive force source(permanent magnets 2 a, 2 b). This is shown most clearly in FIG. 2A.This bifurcation of the static magnetic field at the junction with thecore members, particularly where permanent magnets are used to bridgethe gap between core members, provides protection againstde-magnetisation of the static magnetomotive force source, during afault current event. The arrangement provides common mode rejection ofthe AC field across the poles of the static magnetomotive force source.Such an arrangement is not disclosed in, for example WO2007029224.

The use of one or more pairs of permanent magnetomotive force sources tobridge the gap between core members provided benfits also in that a lowreluctance DC flux path is provided to aid saturation and protection ofthe magnets, whilst at the same time allowing high levels of fluxlinkage through the magnetic circuit to give good current limitinginductance during a fault. The ability to include magnets distributed inthis way provides distributed DC mmf around the magnetic circuit, aidingcore saturation.

The implementation of shielding 4 a, 4 b, for example copper shims orplates (shown in FIGS. 13 and 14) arranged around the electricallyconductive permanent magnet 2 prevents eddy current heating in thepermanent magnet 2 by reducing the direct action of the AC field on thepermanent magnet 2 which also helps to prevent or minimise permanentdemagnetisation. The eddy current effect is represented by the arrowwithin the shielding in FIG. 14.

FIG. 3 shows a core consisting of a series of permanent magnets 2arranged in the form of a ring and sandwiched between two soft magneticring cores 1 a, 1 b. The side edges of the permanent magnets are incontact with the side edge of its neighbour permanent magnet and themagnetic moments (indicated by the bold arrow) of adjacent magnets arein opposite directions.

A coil 3 is wound around the whole core 1 which includes 1 a and 1 bsuch that the direction of AC flux is along the circumferentialdirection of the core 1 and orthogonal to the moments of the permanentmagnets 2. The flux set up by adjacent permanent magnets 2 forms acomplete magnetic circuit through the soft material of the core 1 aboveand below the ring of magnets 2. The relative geometries of thepermanent magnets 2 and core members 1 a, 1 b (in this instance the coremembers are rings of soft core material) are such that the entire core 1is saturated under normal operating conditions by the magnetic circuitprovided by the magnet arrangement, so as to keep the inductance of thecoil 3 at a minimal value. Under fault conditions the saturation isovercome in regions of the core 1 so as to give the core 1 a higher netrelative permeability so as to increase the inductance of the AC coil 3.

For example, in the event of a fault condition the regions in the corewhere the DC/static magnetic field generated by the permanent magnetcoincides with the AC magnetic field, the two fields are additive,thereby causing the core in these regions to saturate even deeper.However, in the regions where the DC/static magnetic field and the ACmagnetic field oppose (i.e. flow in opposite directions) the AC field issubtracted from the permanent magnetic field and the core in theseregions becomes less saturated and may even become unsaturated when themagnitude of the AC fault current is high. The net permeability of thecore 1 is therefore increased causing the inductance of the coil 3 toalso increase so as to prevent and/or limit the passage of faultcurrents through the AC coil 3. Since the AC signal is cyclic (havingpositive and negative half cycles) the AC magnetic field is caused tovary its direction depending on the half cycle and the direction of themagnetic field varies (or alternates) between opposing circumferentialdirections. Therefore, the regions of the core where the DC/static andAC magnetic fields were previously additive are now subtractive and viceversa. The symmetry of the system, therefore enables the core to providethe current limitation for either half of the AC current cycle andeliminates the need for two separate devices each dealing with only onehalf of the cycle (as is the case in FIG. 1).

FIG. 4 shows air gaps 5 arranged between the series of rare-earthmagnets 2 of FIG. 3. Air has a permeability of several orders ofmagnitude lower than the permeability of ferromagnets and ferrimagnetsused for the magnetic cores.

The device operates in the same way as the device in FIG. 3, however theinclusion of the air gaps 5 prevents the magnetic flux from shortcircuiting. The Permanent Magnetic FCL (PMFCL) of FIG. 4 was modelledusing a 3D non-linear transient simulation solver so as to simulate thetransient effect of the PMFCL. The parameters of the model were asfollows:

Firstly Considering the Core:

Inner diameter—200 mmOuter diameter—280 mm

Height—15 mm

Material type—Soft Magnetic silicon steel M4

Next Considering the Permanent Magnet:

No. of poles: 6 equally spacedInner diameter: 200 mmOuter diameter: 280 mm

Height: 30 mm

Material type Neodynium Iron Boron: 34/22Electric resistivity: 1.5e-006 Ωm

Finally Considering the AC Coil:

Number of turns: 20 turnsTotal number of turns in the whole coil: 50Material type: copper

The PMFCL modelled is shown in FIG. 4. The PMFCL is coupled to anelectric circuit in which a defined sinusoidal voltage source isconnected to the PMFCL and load. The circuit is shown in FIG. 5 a. Thecurrent in the circuit is calculated by software according to thecircuit parameters. To simulate the short circuit effect, a switch 6 isconnected in parallel across the load 7. The model is solved when theswitch 6 is open and the current in the circuit is calculated at eachtime step. At a defined instant the switch 6 is closed leading to ashort circuit of the load so as to simulate the fault condition. Thetransient current is determined according to the permanent magnet faultcurrent limiter parameters. The width of the soft steel core 1 of FIG. 4has been selected to be 15 mm so as to achieve full saturationthroughout the volume of the core material.

FIG. 5 b shows a 3D, non-linear, time-step solution for the model ofFIG. 4. The current was calculated at each time step prior to and afterclosing the switch. The graph displays transient current against timefor the PMFCL and an air cored inductor (which is used as a comparisonmodel). The results show a clear attenuation of the circuit current dueto the presence of the PMFCL when compared to the air-cored inductor,whereby the maximum peak is in the region of 25 A compared to 70 A.

FIG. 6 shows a stacked PMFCL arrangement where multiple rings of FIGS. 3and 4 are stacked one on top of the other with the AC coil 3 woundaround the cores 1 in a parallel arrangement. The arrows in FIG. 6represent dipole moments of the respective magnets 2.

FIG. 7 shows a single phase arrangement with a first soft magneticpicture frame shaped core 1 a and a second soft magnetic picture frameshaped core 1 b. The second core is a mirror image of the first core andthey are arranged face-to-face in a parallel configuration. A permanentmagnet 2 a having a magnetic dipole moment perpendicular to the parallelface of the cores is arranged between the first corner of the first core1 a and the first corner of the second core 1 b and a second permanentmagnet 2 b, also having a magnetic dipole moment perpendicular to theparallel face of the cores, is arranged between the second corner of thefirst core 1 a and the second corner of the second core 1 b. A third andfourth permanent magnet 2 c,2 d are also arranged between the third andfourth corners of the first and second cores 1 a, 1 b respectively.

The magnetic moments of adjacent permanent magnets (e.g. the first andsecond magnets 2 a, 2 b) are arranged in opposite directions. The fluxset up by the first and second permanent magnet 2 a, 2 b forms acomplete circuit with the two permanent magnets through the magnetisablecore material 1 i.e. a magnetic field flows from the north pole of thefirst magnet 2 a through the first arm of the first core 1 a to thesouth pole of the second magnet 2 b and a field flows from the northpole of the second magnet 2 b through the first arm of the second core 1b to the south pole of the first magnet 2 a. Therefore the magneticfield flows in opposite directions in the first arms of core 1 and core2 1 a, 1 b. The same process occurs with the remaining arms of thedouble frame structure. The relative geometries of the core sections 1a, 1 b and the permanent magnets 2 a, 2 b, 2 c, 2 d are such that undernormal operating conditions the soft magnetic material of the core 1remains saturated caused by the magnetic circuit generated by thepermanent magnet arrangement.

An AC coil 3 is wound around the first and second cores 1 a, 1 b so asto provide a magnetic field in a direction perpendicular to the magneticmoment of the first, second, third and fourth permanent magnets 2 a, 2b, 2 c, 2 d.

In normal operation the inductance of the coil 3 is low since the firstand second cores 1 a, 1 b are saturated by the DC/static fieldsgenerated by the permanent magnet arrangement. When a fault conditionoccurs a high current (a fault current) flows through the AC coil 3 andthe AC magnetic field generated by the AC coil 3 (that flows through thefirst and second core 1 a, 1 b) increases in magnitude and becomesstrong enough to overcome the saturation in regions of the cores.

For example, firstly considering the effect at the first arm of thefirst and second cores 1 a, 1 b; when considering the positive halfcycle of the AC signal, the magnetic field produced by the AC coil 3 issubtracted from the magnetic field produced by the permanent magnet 2,therefore at least part of the region in the first arm of the first core1 a becomes unsaturated. However, when considering the first arm of thesecond core 1 b during the positive half-cycle of the AC signal thefirst arm of the second core 1 b gets saturated even deeper, with itsimpedance not changing. The overall effect is an increase in inductanceof the AC coil 3 due to a net increase in μ_(r) of the core (inaccordance with equation 1) and an increase in inductance limits thepassage of the fault current through the coil 3. When considering thenegative half of the cycle the region in the first arm of the secondcore 1 b becomes unsaturated and the first arm of the first core 1 abecomes saturated again. Therefore, the overall effect is an increase ininductance of the AC coil 3 due to a net increase in μ_(r) of the core 1(in accordance with equation 1). Consequently, this arrangement respondsto either half of the AC cycle.

The three phase PMFCL has a more complicated configuration of themagnetic core 1. A popular core design for a three-phase transformer isthe three-limb core shown in FIG. 8 having a first section and a secondsection 1 a, 1 b. The AC coils 3 a, 3 b, 3 c are placed on the threelimbs of both sections or the core 1 a, 1 b (shown extending verticallyin FIG. 8).

FIG. 9 shows the core 1 and magnets 2 of the FCL where an air gap 5 isfirstly positioned along the circumferential direction so as to providea space between the adjacent permanent magnets to reduce the DC magneticflux short circuit, whereby the permanent magnets are arranged between afirst and second section of the core. Secondly an air gap 9 ispositioned within the first and second (or upper and lower) core membersat the mid point above and below each permanent magnet i.e. themid-point above the pole faces 8 so as to divide a core section 1 a, 1 binto two further parts. The use of air gaps 9 along the circumferentialdirection of the soft magnetic core advantageously increases the ACfield reluctance of the soft magnetic cores in order to avoid very largeAC fields (which are generated by very large fault currents) fromdriving the core back into saturation which would curtail the currentlimiting liability of the FCL. It is also important not to increase theDC flux path and this can be achieved by placing the air gaps 9 at themid-point above and below the pole faces of the permanent magnets at theregion of DC bifurcation. This feature has been found to greatly enhancethe performance of the device by extending the working fault currentlimiting range.

FIG. 10 shows the air gap 10 extending through the permanent magnet.This has the advantage of minimising the effect of demagnetisationcreated by the free poles in FIG. 9 and thus reducing the reluctance inthe DC flux path, thereby aiding saturation of the soft magnetic coresin the regions between the magnets and also permits the cores to beconstructed in sections so as to aid the manufacturing process.

FIG. 11 shows an FCL core with soft magnetic rings 1 a, 1 b andpermanent magnets 2 arranged as annular rings. The air gaps 5, 9, 10 ofFIGS. 9 and 10 can also be included in this structure. The AC windings(not shown) are wound around the core with the AC field in thecircumferential direction, perpendicular to the magnetic moment of thepermanent magnet.

FIG. 12 shows a section of an FCL core. The central soft magnetic corelc has a thickness double that of the outer cores 1 a, 1 b so as toallow the DC flux density to be equal since the permanent magnets 2above and below act on the central core 1 c. The AC windings (not shown)are wound on the central soft core 1 c only in the spaces 5 between themagnets 2 so as to allow this core structure to be stacked above andbelow.

FIG. 13 shows a section of an FCL core where electrically conductiveplates 4 a, 4 b have been placed either side of a permanent magnet 2,facing the direction of the AC field. The AC windings (not shown) enwrapthe entire cross section of the structure producing a field in thelongitudinal/circumferential direction of the soft magnetic cores,depending on the core structure, or alternatively could be arranged asshown in FIG. 12. Eddy currents would be generated in the plates(represented by the arrow in the conductive plates) which generate apartially cancelling magnetic field in the opposite direction (inaccordance with Lenz's Law) and therefore serves to protect thepermanent magnet from the demagnetising effect of the AC field.

FIG. 14 shows conductive plates 4 being positioned above and below thepermanent magnet 2 in order to shield the magnets from any component ofthe AC flux which may leak from the soft magnetic cores 1 a, 1 b.

FIG. 16 shows a development of FIG. 11 whereby the inner soft magneticring has been reduced to zero. Therefore the magnetisable core memberdefines a void region 12 so as to form an annulus and the permanentmagnet 2 now extends across the void 12 of the annulus 1 such that thenorth and south ends of the permanent magnet make contact with the inneredge of the annulus. The core member is separated into two halves (or afirst and second member) 1 a, 1 b by gaps 13 which extend from theinternal surface of the annulus 1 to the outer surface of the annulus 1at the mid-point of the south and north poles of the magnet, i.e. fromthe mid-point of the pole faces 8. Alternatively, the core need not havegaps or the gap could extend through the full length of the permanentmagnet. The permanent magnet arranged in this way causes at least a partof the first and second members 1 a, 1 b of the core structure (orannulus) to become saturated in normal operation.

The AC windings 3 are configured such that the AC fields “chase” eachother. In such a case, and in the event of a fault condition, one halfof the soft core 1 a is driven further into saturation whilst the otherhalf 1 b is driven towards sub-saturation. Therefore in the event of afault current, one half of the core 1 will be driven out of saturationcausing the inductance to increase so as to limit the fault current to asafe level. The core 1 would react equally to either half of the ACwaveform i.e. the positive half of the AC cycle would overcomesaturation in one half of the magnetisable core and the negative halfcycle would overcome saturation in the second half of the magnetisablecore. The core is capable of resetting automatically following the faultevent. This design can lead to an increase in flux leakage compared toan FCL core where the permanent magnetic material is more spaciallydistributed.

FIG. 17 shows a similar core to FIG. 16, but in this case the ACwindings 3 are placed around the outer circumference of the softmagnetic core 1. The section can be combined with other sections so asto form a torus and can be assembled in sections with an air gap 5between each section. The air gap would increase the reluctance of theAC field direction whilst at the same time not affecting the DC fluxpath.

FIG. 18 shows a cross section through a torus core 1. The DC flux pathacts in an orthogonal direction to the AC flux. Air gaps (not shown) canbe inserted at intervals along the circumferential path length of thetorus 1. A permanent magnet 2 extends from the interior of the torus tothe exterior of the torus throughout the torus structure.

In the arrangements of FIG. 17 and FIG. 18 the DC flux and the AC fluxinteraction is orthogonal rather than parallel and may be more suitedfor applications such as a low loss transformer core. In thisarrangement the domain re-orientation is by rotation only rather thandomain wall movement followed by moment rotation (at low and then highfields respectively).

Hysteresis is associated only with the pinning of domain walls duringwall movement and so this component of loss would not exist or begreatly reduced in the core of this FCL arrangement.

It will be appreciated that in further embodiments various modificationsto the specific arrangements described above and shown in the drawingsmay be made. For example the permanent magnets can be embedded in a coreformed of one single core section, whereby the magnets may be insertedinto slots or other forms of recess.

The core member may be formed of strip steel or other ferromagneticmetal alloy, a soft ferrite material or an amorphous or nanocrystallinesoft magnetic alloy.

The AC coil may be wound around the core in multiple configurations soas to provide an AC magnetic field which is orthogonal to the magneticdipole moment of the core, for example when considering the embodimentof FIG. 2, the coil can also be wound around the first core then thesecond core in a series arrangement or instead the coil may be woundaround both the first and second portion in a parallel arrangementdescribed previously. Also the permanent magnets in the embodiment ofFIG. 2, or equivalent embodiments, may be positioned at any pointbetween the core arms as long as they form a magnetic circuit in thecore. The positioning of the magnet pair may not need to form asymmetrical arrangement. The core can be wound around each arm pair ofthe core or only a single arm pair.

Eddy current heating in the electrically conductive permanent magnetsmay also be prevented by replacing the copper shims with anothershielding means. For example the soft iron core itself may be laminatedor formed using bonded compacted powders. Samarium Cobalt (SmCo)materials can be selected to give better performance at hightemperatures with respect to Neodymium-Iron-Boron based rare earthalloys. Alternatively, when considering the ring arrangement thepermanent magnets can be spaced apart by regions of the magnetic core(and not the magnets) and the AC windings can be wound only over thesoft magnetic cores so as to reduce eddy currents and to reduce thelength of the conductor required in the windings. In a furtherembodiment the rare earth magnet may not require the use of a shieldingmeans.

Disc, toroidal and picture frame core designs have been presented above.The picture frame shaped core can be formed by cutting the core materialor, alternatively a picture frame shaped core can be formed by winding amagnetic material on a former, however this method results in a pictureframe core having more rounded corners in comparison with the previouspicture frame core. Alternative core designs may be used in the presentinvention. The magnetic material may be in the form of a lamination,film, strip, tape, ribbon, wire or the core may be formed by sinteringpressing.

It is clear that other core cross sections are suitable for use in thepresent invention and may include rectangular, circular, cruciform orcross-shaped, orthogonal and triangular. FIG. 15 further demonstratesthat the shapes of permanent magnet 2 may also be varied. FIG. 15 (a)shows the case where the permanent magnets 2 are placed side-by-sidewith no gaps, (b) shows the permanent magnets 2 spaced by the air gaps 5and (c) shows the arrangement where parallel sided gaps 11 are spaced atintervals within a continuous permanent magnet structure 2 b. Differentprofiles may be selected in order to optimise the path of the DC flux inthe soft magnetic cores so as to maximise the volume of soft magneticmaterial which is held at, or near, saturation.

Instead of a stacked arrangement of core members, a chain arrangementcomprising a plurality of core elements, each defining a centralaperture, the core elements being joined together in order to create aclosed magnetic circuit may be applied. The AC coil may be wound aroundeach of the linked arm pairs in a series arrangement. The magneticmaterial may be in the form of a lamination, film, strip, tape, ribbonor wire or the core may be formed by sintering or pressing.

The permanent magnet need not be made from electrically conductingrare-earth materials which possess a good re-coil capability.Alternatively, hard ferrites which are non-conducting oxides can beselected for the permanent magnet. This is a good alternative because itis considerably cheaper than the high energy rare-earth magnets andremoves any need for the shims or plates to be implemented. However thesize of the Fault Current Limiter for a given rating where hard ferritesare implemented would be substantially larger than when electricallyconducting rare-earth materials which possess a good re-coil capabilityare implemented.

When considering FIG. 3 it is clear that the permanent magnets may beplaced side by side such that the magnets are in contact with eachother, however this can provide a lack of DC flux into the soft magneticcore and would require the soft magnetic cores to be thinner in order toget saturation. Therefore by separating the permanent, magnets by airgaps the DC flux follows paths through greater sections of the softmagnetic core. Optimum separation of magnets will depend on magneticcharacteristics of soft and permanent magnetic materials and coregeometry. The second type of air gap, which has a minimal effect on theDC flux can extend through the soft magnetic core sections only or itmay extend through the core sections and the length of the permanentmagnet. The second air gap in the soft core need not be present in theFCL arrangement.

When considering FIGS. 9 and 10 it is clear that wider air gaps may beimplemented, however modelling has shown that the arrangement of the airgaps as shown in FIGS. 9 and 10 may provide a greater advantage to theavoidance re-saturation by very large fault currents. However asdescribed previously there may be air gaps within the core positioned atthe mid-point of every permanent magnet, or alternatively at themid-point of just some. The air gaps may or may not all be the samelength. The width of the air gaps may be a set value, or could bealtered at a later date so as to suit the particular application of theFault current limiter.

A development of the core configuration shown in FIG. 11 could be madein which the AC windings are wound around the circumference of the outersoft magnetic ring and the core (similar to the embodiment of FIG. 17)and whereby the core is assembled in sections similar to that shown inFIG. 16. Further air gaps could be inserted between each section.

In the previous embodiments the AC coil is wound around the completeFault Current Limiter core comprising the soft (the core) and the hard(the permanent magnet) components, however the AC windings can be woundaround the soft magnetic core 1 only in the region of the soft magneticcore where the saturation is to be achieved, rather than just in onelocation or remote from this, or these, regions. This beneficiallyreduces the flux leakage from the soft cores.

FIG. 19 shows a wound construction of the core 1 and magnet structure 2which offers an alternative method of construction in which a moreautomated winding of the core could be envisaged. FIG. 20 shows theconcentric ring construction which is similar to FIG. 11 but in thiscase shows a greater number of core rings 1 having permanent magnetsdistributed around the circumference of the core ring structure. Thearrangement of FIG. 20 offers an improvement of the performance of theFCL by enabling a greater distribution of the DC magnetic field sourcesin the structure which allows a greater volume of the soft magnetic coreto be saturated.

The number of turns of the AC windings in the arrangements shown inFIGS. 11, 19 and 20 can be altered so as to ensure that the H values areequal in the inner and outer cores of the structure which have differingpath lengths. Therefore the number of turns N is tailored to give acertain AC field value (H) for a given magnetic current I and pathlength l, in order to ensure the same flux density B is generated in themagnetisable material in the core sections e.g. inner and outer coreswhich form the core as a whole.

Consideration can be given to the pitch of the winding i.e. theconcentration of the winding at different regions of the core for agiven number of turns of the coil, (whereby windings are placed closertogether in regions that require a stronger magnetic field and windingsare spaced apart at larger intervals where a reduced magnetic fieldstrength is desired). The pitch is varied so as to uniformly take out50% of the core during each half cycle. This variation of the pitch ofthe winding would also avoid regions of the core which are initiallyunder-saturated thereby minimising the coil inductance under non-faultconditions.

In an alternative embodiment the permanent magnets could be replaced byDC conducting or superconducting coils containing a soft magnetic or anair core. This would provide adjustability of the DC static magneticflux. The interaction of the AC and DC fluxes are the same as describedwhen selecting permanent magnet configuration. It is also envisaged thata combination of a DC coil and a permanent magnet can be implementedtoo.

In an alternative embodiment of FIG. 18, additional permanent magnetscould be inserted at positions around the cross sectional circumferenceof the toroidal core, or alternatively the permanent magnet may bereplaced with a poloidal winding along which is provided a DC current soas to generate a static DC magnetic field.

The embodiments of the invention may exist as multilayers of air gappedmagnets and cores so as to form discrete sectors i.e. the core is formedfrom at least two segments which are arranged to form the core as awhole. The sectored, or air gapped core, has constructional benefitsenabling a high current winding to be manufactured in sections orsegments, whereby piece parts of the core sectors and corresponding coilsectors can be assembled to form the completed toroid structure.

The present invention comprises combinations of features described withrespect to different embodiments.

Advantages include that the FCL is always ready as a) it respondsequally to each half of the AC cycle and b) the permanent magnets arenot easily permanently demagnetised, springing back to their originalmagnetic condition after a fault current event has occurred. The FCLgreatly reduces the use of costly materials compared to the prior artand is also operable over a broad power range in single and three phasealternating power systems such that the FCL can be used in low power,(i.e. more numerous) applications and high power applications. Further,the relative orthogonal arrangement between the DC/static magnetomotiveforce source and the varying magnetomotive force source protects thepermanent magnets and aids saturation of the core material since theDC/static magnetomotive force can be more distributed without increasingthe AC reluctance of the core material (since the relative permeabilityof the permanent magnet material is low and is seen by the ACmagnetomotive force as a high reluctance element in the magneticcircuit).

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe capable of designing many alternative embodiments without departingfrom the scope of the invention as defined by the appended claims. Inthe claims, any reference signs placed in parentheses shall not beconstrued as limiting the claims. The word “comprising” and “comprises”,and the like, does not exclude the presence of elements or steps otherthan those listed in any claim or the specification as a whole. Thesingular reference of an element does not exclude the plural referenceof such elements except when specifically stated as such and vice-versa.The mere fact that certain features are recited in mutually differentdependent claims does not indicate that a combination of these featurescannot be used to advantage.

1. A fault current limiter (FCL) comprising: at least one magnetisablecore member; at least one AC magnetomotive force source configured togenerate a varying magnetic flux in at least a portion of the at leastone magnetisable core member; and; at least one static magnetomotiveforce source being positioned to provide a magnetic circuit within atleast part of the at least one magnetisable core member, characterisedin that the AC magnetomotive force source and the static magnetomotiveforce source are relatively positioned to be orthogonal to each other.2. A fault current limiter according to claim 1, wherein a first coremember is provided, and a second core member is provided, spaced fromthe first core member, the AC magnetomotive force source configured togenerate a varying magnetic flux in both of the first and second coremembers.
 3. A fault current limiter according to claim 2, wherein thefirst core member comprises a first closed magnetic circuit and thesecond core member comprises a second closed magnetic circuit distinctfrom the first closed magnetic circuit.
 4. A fault current limiteraccording to claim 3, wherein the first and/or second magnetic circuitare comprised of core members spaced by air gaps.
 5. A fault currentlimiter according to claim 3, wherein the static magnetomotive forcesource is positioned to provide magnetic saturation in both of themagnetic circuits.
 6. A fault current limiter according to claim 2,wherein the static magnetomotive force source is positioned to bridgethe gap between the first and second core members.
 7. A fault currentlimiter according to claim 1, wherein static magnetomotive force sourceis positioned to provide a bifurcated magnetic field in the core memberadjacent to the static magnetomotive force source in which the fielddirection in the same core member branches in opposed directions at thejunction with the static magnetomotive force source.
 8. A fault currentlimiter according to claim 1, wherein the AC magnetomotive force sourceis an AC coil having a longitudinal axis and to which is applied an ACcurrent so as to produce an AC magnetic field.
 9. A fault currentlimiter according to claim 1, wherein the static magnetomotive forcesource is a magnet having a magnetic dipole moment associated with it.10. A fault current limiter according to claim 1, wherein the ACmagnetomotive force source is an AC coil having a longitudinal axis andto which is applied an AC current so as to produce an AC magnetic field,wherein the static magnetomotive force source is a magnet having amagnetic dipole moment associated with it, and whereby the AC coil andthe at least one magnet are relatively positioned such that thelongitudinal axis of the coil is orthogonal to the magnetic dipolemoment of the magnet.
 11. A fault current limiter according to claim 9,wherein the at least one magnet is formed from a permanent magneticmaterial having a default magnetisation.
 12. A fault current limiteraccording to claim 1, wherein the arrangement of the at least one magnetcauses at least part of the at least one core member to becomemagnetically saturated in normal operation.
 13. A fault current limiteraccording to claim 1, wherein in a fault condition the magnitude of theAC magnetic field increases from a normal state value to overcomemagnetic saturation in at least part of the at least one core member.14. A fault current limiter according to claim 7, wherein the AC coilobtains a higher inductance value when a fault condition occurs.
 15. Afault current limiter according to claim 2, wherein the at least onemagnetisable core member responds equally to the positive and negativehalves of the AC current cycle received by the AC coil.
 16. A faultcurrent limiter according to claim 1, wherein the magnetisable coremember contains at least one air gap.
 17. A fault current limiteraccording to claim 1, wherein at least one air gap is arranged betweentwo adjacent permanent magnets so as to space apart the at least twoadjacent magnets.
 18. A fault current limiter according to claim 17,wherein the at least one air gap in the magnetisable core member ispositioned above and below the surface of the pole faces of the at leastone permanent magnet.
 19. A fault current limiter according to claim 18,wherein the at least one magnet is intersected by an air gap.
 20. Afault current limiter according to claim 1, wherein the staticmagnetomotive force source is a permanent and the permanent magnetrecovers its default magnetised state on cessation of a fault currentevent.
 21. A fault current limiter according to claim 1, wherein the ACmagnetomotive force source is an AC coil and the AC coil is wound aroundthe region of the magnetisable core member where the overcoming ofsaturation is to be achieved.
 22. A fault current limiter according toclaim 21, wherein the AC coil is wound around the outer circumference ofthe magnetisable core member.
 23. A fault current limiter according toclaim 1, wherein there is included shielding to minimise the effects ofeddy currents or demagnetisation of the static magnetomotive forcesource.
 24. A fault current limiter according to claim 1, wherein the atleast one magnetisable core member is formed of strip steel or otherferromagnetic metal alloy, a soft ferrite material or an amorphous ornanocrystalline soft magnetic alloy.
 25. A fault current limiteraccording to claim 1, wherein the core member is formed from at leasttwo core segments which are arranged to form the core member as a whole.26. A circuit including the fault current limiter of claim
 1. 27. Analternating current power system including the fault current limiter ofclaim
 1. 28. A fault current limiter accorder to any claim 1 wherein themagnetisable core member has a void defined therein.
 29. A fault currentlimiter comprising: at least one magnetisable core member having a voiddefined therein; at least one AC magnetomotive force source configuredto generate a varying magnetic flux in at least a portion of the atleast one magnetisable core member; and at least one staticmagnetomotive force source being positioned to provide a magneticcircuit within at least part of the at least one magnetisable coremember characterised in that the at least one static magnetomotive forcesource is arranged to extend across the void.
 30. A fault currentlimiter according to claim 29, wherein the magnetisable core member isprovided with a first half and a second half, and the AC magnetomotiveforce source is an AC coil wound around each half of the magnetisablecore member which defines the void.
 31. A fault current limiteraccording to claim 29, wherein at least two air gaps are included tosection the core member into first and second spaced core members.
 32. Afault current limiter according to claim 31, wherein the arrangement ofthe at least one static magnetomotive force source causes at least partof the first and second spaced core members to become saturated innormal operation.
 33. A fault current limiter according to claim 32,wherein saturation in one of the magnetisable core members would beovercome by the positive half of the AC cycle and saturation in thesecond of the magnetisable core members would be overcome by thenegative half of the AC cycle.
 34. A method of preventing a faultcondition in an AC magnetomotive force source positioned relative to atleast one magnetisable core member having at least one staticmagnetomotive force source arranged such that the AC magnetomotive forcesource and the static magnetomotive force source are relativelypositioned to be orthogonal to each other, the method comprising: usingthe static magnetomotive force source to create a magnetic circuitwithin at least part of the at least one magnetisable core member so asto saturate at least part of the magnetisable core member, therebyproviding a low inductance for the AC magnetomotive force sourceenabling current to flow along the AC magnetomotive force source innormal operation; producing an AC magnetic field which overcomes thesaturation in at least part of the at least one magnetisable core memberin the event of a fault condition; and increasing the inductance of theAC magnetomotive force source in the event of a fault condition so as tolimit the passage of fault current flowing therethrough.
 35. A methodaccording to claim 33 wherein the fault current limitation respondsequally to either half of the AC cycle.
 36. A method of preventing afault condition in an AC conductive element wound around at least onemagnetisable core member having at least two magnets with opposingdipole moments arranged within the at least one magnetisable coremember, the method comprising: creating a magnetic circuit within atleast one magnetisable core member; saturating at least part of the atleast one magnetisable core member so as to provide a low inductance forthe conductive element enabling current to flow along the conductiveelement in normal operation; producing an AC magnetic field whichovercomes the saturation in at least part of the at least onemagnetisable core member in the event of a fault condition; andincreasing the inductance of the conductive element so as to limit thepassage of fault current flowing therethrough, characterised in that themagnetic field generated by the conductive element is orthogonal to themagnetic dipole moment of the at least two magnets.
 37. A methodaccording to claim 35 wherein the fault current limitation respondsequally to either half of the AC cycle.
 38. A method of preventing afault condition in an AC magnetomotive force source positioned relativeto at least one magnetisable core member having a void defined thereinand having at least one static magnetomotive force source arranged toextend across the void, the method comprising: using the staticmagnetomotive force source to create a magnetic circuit within at leastpart of the at least one magnetisable core member, so as to saturate atleast part of the magnetisable core member, thereby providing a lowinductance for the AC magnetomotive force source and enabling current toflow along the AC magnetomotive force source in normal operation;producing an AC magnetic field via the AC magnetomotive force sourcewhich overcomes the saturation in at least part of the at least onemagnetisable core member in the event of a fault condition; andincreasing the inductance of the AC magnetomotive force source in theevent of a fault condition so as to limit the passage of fault currentflowing therethrough.
 39. A method according to claim 38 wherein thereis included at least two air gaps to segment the magnetisable core intoat least a first and second half wherein saturation in one half of themagnetisable core would be overcome by the positive half of the AC cycleand saturation in the second half of the magnetisable core would beovercome by the negative half of the AC cycle.
 40. A fault currentlimiter (FCL) comprising: at least one magnetisable core member; atleast one AC magnetomotive force source configured to generate a varyingmagnetic flux in at least a portion of the at least one magnetisablecore member; and; at least one static magnetomotive force source beingpositioned to provide a magnetic circuit within at least part of the atleast one magnetisable core member.